PROJECT SUMMARY/ABSTRACT The neural mechanisms that mediate the decline of brain performance with aging are poorly defined and affect many aspects of normal aging life: reductions in motor dexterity, sensory discrimination, executive function, and attention which impact the degree of independence, number of injuries, and fatal accidents. We will define mechanisms of age-related changes in synaptic plasticity and investigate their impact in memory and learning. Our hypothesis is that in the aged cerebral cortex, disruption of the excitation/inhibition balance at the level of the microcircuits of layer 5 (L5) pyramidal neurons leads to reduced formation of long-lasting stable synapses between excitatory neurons, resulting in impaired learning. We have recently described that dendritic spine density of aged mice is stable, but that their dynamics are elevated in somatosensory cortex. But, we do not if density and dynamics of dendritic spines are differentially affected by age in different brain areas. Also, the mechanisms underlying the alteration in synaptic dynamics in the aging brain are unexplored. One possibility is that the intracortical inhibition controlling synaptic plasticity in the adult brain is released with aging allowing the formation of excess synaptic contacts, many of them meaningless and subsequently be eliminated and making the handling and storing of information less effective. Thus, increasing levels of intracortical inhibition in the aged brain may prevent alterations in synaptic dynamics and preserve brain performance. We will test the following hypotheses: (a) elevated dendritic spine dynamics in the aged brain impedes the creation of memory-forming synaptic contacts and impairs the ability of cortical circuits to store/manage information; (b) age- related reduction in inhibitory transmission at the level of the local circuitry of L5 pyramidal neurons is responsible for the increased instability of dendritic spines; (c) restoring intracortical inhibition in the primary motor cortex of aged mice will stabilize dendritic spines of L5 pyramidal neurons and improve performance in a motor learning task. We will use transgenic mice for in vivo 2PE microscopy and optogenetics in the conditional expression of viral vectors, behavioral tasks, and electrophysiological recordings of synaptically connected neurons: Aim 1 will determine that the alteration of synaptic dynamics in the aged brain is a maladaptive mechanism impairing learning. Aim 2 will identify age-dependent changes in PV and SOM neurons of the L5 cortical microcircuit responsible for instability of dendritic spines in pyramidal neurons and impaired learning. Aim 3 will confirm that the age-related decrease of inhibition in L5 pyramidal neurons impairs synaptic plasticity and learning. By using state-of-the-art techniques and innovative experimental approaches will elucidate the effects of normal aging on the assembly and maintenance of cortical circuits to facilitate future development of therapeutic interventions designed to delay the onset of aging-related brain decline and prolong the quality of life and welfare of the elderly. Results from the proposed research may be applied and used for studies on other neurodegenerative disorders.